This invention concerns detecting defects occurring in nonwoven webs and more particularly concerns a web inspection system and method for detecting and extracting signals by an abrupt change in web density from a noisy product scan signal characterstic of good product.
This defect monitoring system is particularly suitable for webs manufactured by the process taught by Kinney in U.S. Pat. No. 3,338,992 but is not restricted thereto. The type of web defects to which this device is particularly applicable includes those formed by a malfunctioning pneumatic laydown jet as it deposits filaments onto a receiver. These defects are sometimes referred to as "blow arounds" or "jet hangs" and are formed when a jet either forces a mass of filaments to be deposited away from its normal location or when a mass of fibers temporarily holds up in the jet and is then deposited in a dense mass on the web-forming filament receiver. All of the defects of this type are characterized by an abrupt change in web density with the resulting appearance of a sharply defined light or dark blotch when viewed either by transmitted or reflected light. Since an acceptable web with no defects has by its very nature of randomly deposited filaments, a short range, uneven structure, a signal produced by a conventional electro-optical scanner such as disclosed in U.S. Pat. No. 3,866,054 of this acceptable web will contain a high level of rapidly changing peaks and valleys. This high background noise signal has in the past effectively masked the onset of the abrupt edge defects even when analyzed by discriminator circuitry employing conventional differentiation. Thus abrupt edge defects have not been detectable by known automatic inspection devices heretofore even though flaws of this type are easily identifiable visually because of their relatively large size and contrast with the surrounding material.